A Trans-Neptunian object (TNO) is any object in the solar system that has an orbit beyond that of Neptune. The Kuiper belt and Oort cloud are names for some subdivisions of that volume of space. Pluto is a trans-Neptunian object, and if it had been discovered today, it might not have been called a planet. Another of the named trans-Neptunian Objects is Varuna. In July 2001 a new object "2001 KX76" half the size of Pluto was discovered. In October 2002 an object larger than "2001 KX76" was discovered, it is named Quaoar.

Gravity, the attracting force between all matter, also attracts planets. Due to the changes in the orbits of the known planets in the early 1900s, it was assumed that there was one or more planets beyond Neptune, but not yet identified. This search for these led to the discovery of Pluto, and since then a few other significant objects have been found. These are still too small to explain the perturbations, though, and revised estimates of Neptune's mass showed that the problem was fictitious.